1. Field of the Invention
The present invention relates to a hydraulic control circuit for an automatic transmission and, more particularly, to a hydraulic control circuit for effecting both the transient pressure control of frictional engagement means and the slip control of a lockup clutch during shifting by a single pressure regulator valve.
2. Description of the Related Art
In a vehicle having an automatic transmission, the engine power is transmitted to the automatic transmission through a fluid coupling such as a torque converter. In order to improve the mileage and absorb any fluctuation in torque, a lockup clutch is interposed in the fluid coupling such that it may either be fully engaged or applied with slipping, as disclosed in Japanese Patent Laid-Open No. 80857/1990. In the case of a lockup clutch applied with slippage, the hydraulic control circuit is usually constructed to include: a lockup relay valve for applying or releasing the lockup clutch by switching the oil pressure between the application side oil chamber and the release side oil chamber of the lockup clutch; lockup switching means for switching the lockup relay valve; a lockup control valve for increasing the difference between the pressure in the application side oil chamber and the oil pressure in the release side oil chamber at the time of application of the lockup clutch, as a signal pressure rises; and a lockup controlling pressure regulator valve for regulating the signal pressure so that the lockup clutch may come into a predetermined state of slippage.
The automatic transmission is equipped with a plurality of gear stages to be selected by applying and releasing hydraulic type frictional engagement means such as clutches and brakes, and with a hydraulic control circuit including a transient pressure control valve for controlling the transient pressure at the time of application and release of the hydraulic type frictional engagement means so that the shock at the time of shifting may be reduced. The transient pressure is controlled according to the signal pressure which is regulated by a transient pressure controlling pressure regulator valve. When the state of the hydraulic type frictional engagement means is switched by operating the shift lever, as in the case of moving the shift ever from a P (parking) range to an R (reverse) range or from an N (neutral) range to a D (drive) range, the shifting shock is also reduced as at the time of shifting by controlling the transient pressure with the transient pressure control valve. The pressure regulator valve for controlling the transient pressure and the pressure regulator valve for the lockup control are each described in the prior art, as in the aforementioned hydraulic control circuit of Japanese Patent Laid-Open No. 80857/1990. The provision of separate pressure regulator valves makes the hydraulic control circuit complex and large-sized and raises the production cost. The pressure regulator valve is either a linear solenoid valve or a duty solenoid valve. Where a linear solenoid valve is used, a more accurate pressure regulation can be achieved, but the above-specified problem becomes more serious because the valve per se is large-sized and expensive.
In view of the foregoing it has been proposed that both the transient pressure control and the slip control of the lockup clutch be provided by a single pressure regulator valve, as disclosed in Japanese Patent Laid-Open No. 253049/1990. Since, in this case, the transient pressure control and the lockup clutch application control have to be temporarily overlapped to prevent racing of the engine at the time of an upshift, the slip control of the lockup clutch is effected exclusively at the highest gear stage where no transient pressure control is required. Specifically, a signal pressure output from the single pressure regulator valve is continuously supplied to both the transient pressure control valve and the lockup control valve, but the lockup control valve is connected to the lockup clutch only at the highest gear stage to control the pressure in the release side oil chamber. At the remaining gear stages, the oil passage to the lockup clutch is blocked to disable the regulating action of the lockup control valve but enable the regulating action of the transient pressure control valve.
However, if the slip control cannot be achieved at the gear stages other than the highest one, a sufficient mileage improving effect cannot be achieved by the slip control. Since, moreover, the lockup clutch is directly switched between the released state and the fully applied state at the gear stages other than the lighest one, the engine R.P.M. and the transmission torque may abruptly change to cause a shock.